An example of repeated farming destroying the soil in even a low-tech society would be the pueblo indians, who came to suffer from the epigenetic effects of poor nutrition a few centuries into settlement of one of their most famous archaeological sites. Were one to line-up well-preserved dead puebloans ordered by date of death, one would find subsequent generations becoming scrawnier and bone structures become more... frail, I suppose. Some dentist wrote about this a hundred years ago. Said dentist also noted more-permanent consequences in the bone structure of other societies who went through, like the puebloans, hundreds of years of this... or, in other words, if you can better adapt to limited nutrition, then not epigenes but instead genes granting scrawnier builds might come to be favored and selected for. It's like island dwarfism, which is also observable in the archaeological remains of prehumans... a silly person once hypothesized this would eventually lead to humans evolving to fill niche roles like those currently occupied by scavengers.
As an aside, though earlier comments came in two years ago, the main practical reason to be careful with migration is that the carbon dioxide footprint of migrants increases tenfold or worse when they move to the first world, among other inefficiencies that are often introduced to the uplofted population unnecessarily. To quote some guy that reminded me of a cross between Freeman Dyson and Vannevar Bush writing in the late 50s, "Instead of giving <west african country> bags of portland cement, which they can't make on their own and need to import from USA, instead try choosing any of about 20 different cement or cement-equivalent formulations <population> can make on their own for free because they clearly own their own land." Or, my population has no overpoulation problem and our home is ridiculously fertile, so maybe stop feeding the people living on poor land, because left to their own devices their land can't support so many... with the exception of the rare place like Ethiopia, I guess. Or, equivalently, kill all Americans who eat avocados in order to flatten the avocado black market, with the same net effect on global climate change. Either would work! As a tangentially related aside, the best way to move to the first world is to purchase citizenship from Vanuatu for $150,000; all Vanuatuan citizens have indefinite time limit and visaless access to all of the Europe Uniion unrestricted by the Schengan Zone. This is approximately how much smugglers charge anyway, and you can even pay online! It's legit and they might even have coupons during the holidays.
EDIT Where is all their plastic waste going....
